This invention relates to a distance measuring method and a distance measuring apparatus and, more particularly, it relates to a distance measuring method and a distance measuring apparatus for measuring the distance to a target by shooting the target with an optical pulse and measuring the time required for the optical pulse to go to and return from the target.
In the above identified technological field, there has been known methods for detecting the arrival time of a received optical pulse from a target by detecting the intersection of the received signal and a signal obtained by delaying it (referred to as self-reference type method).
U.S. Pat. No. 5,726,742 discloses such a self reference type method.
With the method of U.S. Pat. No. 5,726,742, a received signal is divided into two signals and one of them is delayed by means of a delay element. Then, the other one of the two signals is compared with the delayed signal by means of a comparator and the intersection of the two signals is used as reception signal.
However, the method of U.S. Pat. No. 5,726,742 of comparing the received signal with a delayed signal by means of a comparator is accompanied by a problem that the comparator is required to operate beyond its normal operation range particularly when the received signal shows large fluctuations of level or becomes saturated to make it no longer possible to accurately measure the distance.
On the other hand, distance measuring apparatus to be used for avoiding collisions of cars are required to observe the distance separating two cars within a range between 1m representing a high possibility of collision and at least 100 m. Then, the intensity of the received light may vary over more than 5 digits in view of possible variations in the reflectivity of the target.
The cost of processing such signals by means of an ordinary signal processing circuit without saturation will be prohibitive if technologically possible.
Additionally, the delay element that is used for delaying signals can prove itself an obstacle when an application specific integrated circuit (ASIC) is used for the signal processing circuit.